This invention relates to the roughening of a smooth planar metal surface and particularly to the roughening of an aluminum lithographic printing plate substrate by brush graining of an aluminum web.
The art of lithographic printing depends upon the immiscibility of oil and water and the preferential retention of an oily image-forming ink on the image areas and an aqueous dampening fluid on the non-image areas. The lithographic printing plate has a radiation sensitive oleophilic coating that is applied and adhered to the aluminum substrate. When exposed to the imaging radiation and developed, the oleophilic coating remains in the areas to be inked and is removed in the non-printing areas. Where the coating is removed, the hydrophilic substrate is revealed.
In the preparation of a lithographic printing plate substrate, it is desirable to provide a surface which is adherent to the coating, which is highly hydrophilic and which is abrasion resistant. This usually involves anodizing the surface of the aluminum which is sometimes followed by a silicating treatment. The anodizing of the aluminum substrate for lithographic printing plates is well known and, for example, is described in U.S. Pat. No. 3,181,461.
Prior to anodizing, it is a common practice to grain the surface of the aluminum substrate. This is a surface roughening operation which increases the surface area, promotes adhesion of the coating to the substrate and provides for an increased water retention capacity. One such graining technique is rotary brush graining in which a slurry of fine graining particles abrade the surface of the aluminum under the action of rotary brushes. For a teaching of rotary brush graining, see U.S. Pat. No. 4,183,788. In a typical operation an aluminum web is moved through the graining apparatus at the rate of perhaps 100 to 120 feet per minute. A series of brushes having nylon bristles about two inches long are mounted transversely across the web such that the bristles engage and move over the web in the direction of travel of the web. The slurry of graining particles, typically aluminum oxide, is delivered onto the web prior to the brushes. The brushes are rotated at speeds which produce a velocity at the tip of the bristles of up to 2,000 feet per minute or more. The action of the brushes over the surface of the web moves the graining particles along the web surface in the direction of the movement of the web. The result of this arrangement for rotary brush graining is that there is a propensity or proclivity for the roughening to form minute ridges or striations extending along the web in the direction of web movement. In addition to being visibly objectionable, the surface topography is less than ideal in terms of peaks and valleys. A mapping of the surface topography by optical interferometry showing the ratio of peaks to valleys as a function of the mid point between the highest peaks and lowest valleys indicates a preponderance of peaks. This is detrimental to water retention, which occurs in the valleys, and to the ink/water balance. It also requires thicker plate coatings to cover the peaks and retain that coverage as the coatings wear down during usage.
The object of the invention is to brush grain the aluminum substrate of a lithographic printing plate in a manner which will reduce striations or ridges produced by prior art brush graining. The invention involves brush graining with a plurality of brushes with the track of the movement of the bristles of at least one brush over the web surface being at an angle to the track of the movement of the bristles of one or more other brushes over the web surface.